Porous organic polycarbene nanotrap for efficient and selective gold stripping from electronic waste

The role of N-heterocyclic carbene, a well-known reactive site, in chemical catalysis has long been studied. However, its unique binding and electron-donating properties have barely been explored in other research areas, such as metal capture. Herein, we report the design and preparation of a poly(ionic liquid)-derived porous organic polycarbene adsorbent with superior gold-capturing capability. With carbene sites in the porous network as the “nanotrap”, it exhibits an ultrahigh gold recovery capacity of 2.09 g/g. In-depth exploration of a complex metal ion environment in an electronic waste-extraction solution indicates that the polycarbene adsorbent possesses a significant gold recovery efficiency of 99.8%. X-ray photoelectron spectroscopy along with nuclear magnetic resonance spectroscopy reveals that the high performance of the polycarbene adsorbent results from the formation of robust metal-carbene bonds plus the ability to reduce nearby gold ions into nanoparticles. Density functional theory calculations indicate that energetically favourable multinuclear Au binding enhances adsorption as clusters. Life cycle assessment and cost analysis indicate that the synthesis of polycarbene adsorbents has potential for application in industrial-scale productions. These results reveal the potential to apply carbene chemistry to materials science and highlight porous organic polycarbene as a promising new material for precious metal recovery.

in DFT calculations. 7. The saturated adsorption capacity of this manuscript is 2.09 g/g. If the author carefully read more recent literatures, it can be found that adsorption performance of the proposed POPcarbene adsorbent is not superior in the field of gold recovery. 8. The preparation of Ptraiz-CN-A powder material is rather complicated and expensive, lack of superiority in the engineering application.
Reviewer #4 (Remarks to the Author): In this study, it has been studied on the a poly(ionic liquid)-derived porous organic polycarbene (POPcarbene) adsorbent with superior gold-capturing capability. It has been also utilized theoretical calculations by Density Functional Theory. Study and its results are interesting and publishable after the authors should address the following comments.
1) Line 59 on page : Authors stated the reference of 15 as "Yavuz et al", however, that reference is not a reference by "Yavuz et al", so all references should be checked and corrected.
2) Las part of the Introduction section has some sentences about results/discussion/conclusion; this part should only have the aims of the study, so it should be corrected.
3) Some references should be given for the sentence located at line 127 on page 6. 4) Bigger geometries for the mechanism steps stated on Figure 5 should be given in supporting information. 5) Authors stated at line 30 on page 17 that "… in spite of the negligible energy barrier in step 2." However, no further information about on the transition state calculations, details for the TS calculations, geometry, activation barrier value, characterization of the TS geometry etc., These should be stated in the manuscript. 6) What are the spin multiplicity values for DFT calculations in Gaussian? If the spin multiplicity values greater than 1 (singlet) α and β molecular orbitals should be must be taken into attention. 7) Some error, called as Spin Contamination <S2>, may be introduced into the calculations where spin multiplicity is utilized. The spin contamination value must be negligible (less than 10%, David C. Young, Computational Chemistry, 2001 John Wiley & Sons, Inc. page 228). Thus, related with (1), <S2> values should be given in text. 8) Spin Density values might be given for the atoms (especially gold atoms) on the structure. These values tell us where unpaired electrons are located in the system. 9) Some other critical values such as chemical hardness, chemical potential, electronegativity may calculated and used to comparison for activities These values can be easily calculated by using HOMO and/or LUMO values based on the approximation of Koopmans. 10) How did you characterized the geometries obtained by DFT calculations? 11) Vibrational Infrared frequencies can be calculated. They can be compared with the experimental values stated on page 7, and they can be used to characterize the geometries. Additionally, some mentions about the factor that will be used to scale the frequency values should be stated in text. (Frequency values should be scaled to reproduce experimental fundamentals, the factor and its reference(s) should be stated in text). 12) What are the convergence criteria in calculations? Gradients of root-mean-square (rms) displacement, max displacement, rms force, max force and the self-consistent field (SCF) convergence. 13) NBO analysis should be utilized on the geometries and charge values should be mentioned by using the experimental findings. 14) Some DFT calculations about solvent effect should be utilized and results should be compared with present values. 15) Do the energy values include Zero Point Energy (ZPE) corrections? If it does not contain, ZPE should be calculated and inserted into energy values or any comments on ZPE should be inserted into the related text 16) Energy values should be included thermal energy corrections. 17) Density-of-states and Partial Density-of-states and electronic configurations can be calculated in order to compare results. 18) Molecular Electrostatic Potential diagrams and HOMO and LUMO representations can be obtained and they can be compared with after/before adsorption. 19) Some comment for BSSE can be inserted into text.
A barely changed before and after water infiltration (Fig. R2).
After that, we further modified our supercritical CO2 drying process and acquired Ptriaz-CN-A with a specific surface area of 332 m 2 /g (Fig. R3). Such material with higher porosities exhibited an even faster water infiltration compared to Ptriaz-CN-A with a relative lower surface area of 177 m 2 /g (6 min vs. 11 min), and the infiltration process is also reversible (Fig. R4). The results testify that the hydrophilicity of Ptriaz-CN-A could be easily achieved via a short time of water contact. Fig. S5 shows that the surface layer has a water contact angle of about 60 o initially, and after 6 min a spontaneous infiltration process occured, indicating that the adsorbent in contact with water becomes superhydrophilic, and the infiltration process is reversible. The Scanning electron microscopy (SEM) characterization also revealed that the microstructure of the Ptriaz-CN-A barely changed before and after water infiltration (Fig. S6). Meanwhile, chemical process was also considered and evidenced by time-dependent 1 H-NMR test, the results proved that C5-proton is highly active, and a reversible proton exchange take place between H2O and Ptriaz-CN-A, resulting in a time-dependent interaction of Ptriaz-CN-A with water ( Fig.   S7)."   (2010).). Furthermore, the protonated amidine may arise because the H in the amidine will be removed by the excess NH3 in the solution. To further investigate the chemical structure of samples, the 13 C NMR spectra were measured for typical TriazoleTFSI with or without ammonia treatment. As shown in Fig. R5b, carbon signals at 163 ppm (Chem. Mater. 22, 5492-5499 (2010)), 166-169 ppm (Macromolecules 53, 10366-10374 (2020)) and 170 ppm (Angew. Chem. Int. Ed. 57, 8438-8442 (2018)) were observed for ammonia treated TriazoleTFSI-NH3, but absent in the pristine TriazoleTFSI, which belong to the protonated amidine, conventional amidine and s-triazine, respectively. The above results prove that both striazine and amidine structures will exist in the ammonia-treated TriazoleTFSI. However, different from monomer TriazoleTFSI, in Ptriaz-CN (the polymerization product of TriazoleTFSI), characteristic peak of amidine in FT-IR and solid-state 13 C NMR spectra are not obvious and cannot be directly distinguished from other signals. In contrast, characteristic peak of s-triazine is obviously seen and can be easily pointed out (Fig. R6) (ACS Macro. Lett. 6, 1-5 (2017) & Mater. Horizons 7, 2683-2689 (2020)). These results provided evidence that due to the "cation-methylene-nitrile"sequence, the polymer Ptriaz-CN would make crosslinking via nitrile cyclization under ammonia treatment to form s-triazine structures that covalently cross link the polymer (Mater. Horizons 7, 2683-2689 (2020)). Meanwhile, according to the evidence we obtained from the tests done to the monomer TriazoleTFSI, the amidine structures in the crosslinked polymers after ammonia treatment also cannot be simply ruled out. As a consequence, we believe both s-triazine and amidine structures coexist in the Ptriaz-CN-A, although the s-triazine structure may be the dominant in the polymer product. Hence, we revised the descriptions and schemes in the manuscript to clarify Reviewer 1's concern.

Revisions made, supplementary information (Page 11 line 214): new
Also, our previous work (ref 23) employs ammonia to make membranes without any claim for s-triazine formation. This is because the mechanisms behind the porous membrane formation were complex, involving both ionic crosslinks (obvious to us from the right beginning) and the s-triazine formation (not easy to identify, as nitrile's trimerization occurs usually under much harsher conditions). So, the crosslinking mechanisms were not fully revealed and neither thoroughly investigated (Nat. Commun. 9, 1717 (2018)). Two years after our work, one of our colleagues raised this question and clarified the unusual s-triazine formation mechanism very recently (Mater. Horizons 7, 2683-2689 (2020)). Based on what has been achieved, we believe that the "cation-methylenenitrile" sequence in the PIL used would make crosslinking via nitrile cyclization under ammonia treatment to form s-triazine structures, similar to the paper published in Mater. Horizons 7, 2683-2689 (2020). Thus, both ionic and covalent crosslinking occur in our porous membrane systems.

Revisions made, main text (Page 7 line 142):
"In order to further explore the chemical structure of crosslinked network, we first took unpolymerized precursor-the TriazoleTFSI monomer and and NH3 treated TriazoleTFSI (termed as TriazoleTFSI-NH3) to give sufficient spectroscopic evidence for the formation of networks. As shown in the Fourier transform infrared (FT-IR) spectra of two substances (Fig. S8a), compared with untreated TriazoleTFSI, the appearance of new absorption bands located at 1672 cm -1 (C=N) and 1225 cm -1 (C-N) support the cyclization reaction of nitrile groups into s-triazine ring. In addition, the TriazoleTFSI-NH3 monomer also exhibits the stretching vibration modes of C=N (1610 cm -1 ) for amidine, and 1642 cm -1 for protonated amidine 29 . Furthermore, the 13 C NMR spectra were measured for typical TriazoleTFSI with or without ammonia treatment. As shown in Fig. S8b, carbon signals at 163 ppm, 166-169 ppm and 170 ppm were observed for ammonia treated TriazoleTFSI-NH3, but absent in the pristine TriazoleTFSI, which belong to the protonated amidine, conventional amidine and s-triazine, respectively 29-31 . The above results prove that both s-triazine and amidine structures will exist in the ammonia-treated TriazoleTFSI. However, different from monomer TriazoleTFSI, in Ptriaz-CN (the polymerization product of TriazoleTFSI), characteristic peak of amidine in FT-IR and solid-state 13 C NMR spectra are not obvious and cannot be directly distinguished from other signals. In contrast, characteristic peaks of s-triazine are obviously seen and can be easily pointed out (Fig. 1e & Fig. 1f) 26,32 . These results provided evidence that due to the "cation-methylene-nitrile"-sequence, the polymer Ptriaz-CN would make crosslinking via nitrile cyclization under ammonia treatment to form s-triazine structures that covalently crosslink the polymer 26 . Meanwhile, according to the evidence we obtained from tests done to the monomer TriazoleTFSI, the amidine structures in the crosslinked polymers after ammonia treatment also cannot be simply ruled out. As a consequence, we believe both s-triazine and amidine structures coexist in the Ptriaz-CN-A, although the s-triazine structure is dominant in the polymer product.   R7, the results indicate that there is much room to reduce the environmental impact and cost of Ptriaz-CN-A production when they are produced at scale, and show emerging potentials for industrial productions. Furthermore, the production cost for synthesis of 1 g Ptriaz-CN-A in Scale-1 and Scale-2 is around 117.0 CNY and 107.3 CNY, respectively. The value of the gold captured by 1 g of Ptriaz-CN-A was approximately 795.2 CNY (gold price: about 380.5 CNY/g), and the profit margin will continue to increase by regenerating Ptriaz-CN-A. Therefore, Ptriaz-CN-A contributes a green and sustainable method for gold extraction from e-waste solution. Fig. 6a and Fig.6b was added in main text. Production cost distribution of raw materials, electricity in each step for the synthesis of 1 g Ptriaz-CN-A at different preparation scales. (Scale-1: small dose feeding, data based on our current research; Scale-2: big dose feeding, data based on the maximum production scale of the laboratory.) (Fig. 6 in revised manuscript)

Revisions made, main text (Page 19 line 408):
"LCA is a systematic tool for determining the environmental impact of a product or a process across its entire life cycle or a portion of its life cycle 50, 51 . So, the environmental impacts of Ptriaz-CN-A production prepared at different scales in this research were analyzed using the "cradle-to-gate" LCA approach (The detailed data and analysis are summarized in Additional Supplementary Information-Life Cycle Assessment and Cost Analysis). Fig. AS1 shows the system boundary for this LCA study and the whole data was at the labscale. As shown in Fig. 6a, it is clear that the Ptriaz-CN-A prepared at Scale-2 has much lower environmental impacts across all categories. This is due to the reason that equipment can be operated under full load conditions and resources can be maximized. As a consequence, there is much room to reduce the environmental impact of Ptriaz-CN-A production when they are produced on industrial scale. Cost analysis of raw materials and electricity in Ptriaz-CN-A preparation process in Scale-1 and Scale-2 are presented in Fig. 6b (The detailed data and analysis are summarized in Additional Supplementary Information). The production cost for synthesis of 1 g Ptriaz-CN-A in Scale-1 and Scale-2 is around 117.0 CNY and 107.3 CNY, respectively, and more than 95 % of which are spent on raw materials. Although this production cost seems high, it should be noted that the cost here is based on our laboratory data. It is clear that the cost preparation is reduced when we scale up production under laboratory conditions (Scale-2). Therefore, the preparation cost of this adsorbent can be greatly reduced in industrial production. Furthermore, the value of the gold captured by 1 g of Ptriaz-CN-A was approximately 795.2 CNY (gold price: about 380.5 CNY/g), and the profit margin will continue to increase by regenerating Ptriaz-CN-A. In general, Ptriaz-CN-A contributes a green and sustainable method for gold extraction from ewaste solutions.
Comment 5: 100 ppb is not an "ultra-trace concentration". Reply: We apologize this inaccurate expression. And we changed "ultratrace concentration" to "trace concentration" in the manuscript.

Comment 6:
The porosity (BET area of 177 m 2 /g) is reported without any isotherms or calculations. Figure 1c is not showing any N2 isotherms. BET range and calculations must be disclosed.

Reply:
We thank Reviewer 1's valuable comments and suggestions. We have improved the preparation process. Specifically, we extended the time of crosslinking & solvent exchange process from 2 hours to 4 hours, and in the process of supercritical CO2 drying process, the liquid CO2 cleaning time was extended to 10 h to completely remove the residual DMF in the Ptriaz-CN-A. After certain adjustment of preparation methods, the specific Brunauer-Emmett-Teller (BET) surface area of Ptriaz-CN-A, evaluated with N2 adsorption isotherm obtained at 77 K, is calculated to be 332 m 2 g -1 according to BJH model (Fig. R3).
As for the reported BET area, we believe there is a misunderstanding. Here we marked the adsorption-desorption curve in N2 isotherms in Fig.   R3. Typically, the Ptriaz-CN-A is cooled, under vacuum, to cryogenic temperature (using liquid nitrogen). Nitrogen gas (as a typical adsorbate) is dosed to the Ptriaz-CN-A in controlled increments. After each dose of nitrogen gas, the relative pressure (P/P0) is allowed to equilibrate, and the volume of nitrogen adsorbed is determined. Generally, Brunauer-Emmett-Teller (BET) equation is used to calculate the surface area of solid or porous materials, the BET equation can be described mathematically as follow: is saturation pressure of adsorbate, P is equilibrium pressure of adsorbate, C is BET constant and Vm is the monolayer absorbed gas volume.
As for the porosity (BET area of 332 m 2 /g) of Ptriaz-CN-A, it was calculated by BET model, which described the quantity of adsorbed gas as a function of the relative pressure. Moreover, the detailed information about isotherms and BET range reports, which are calculated and exported by Micromeritics software, are shown in Fig. R8.  Reply: We thank Reviewer 1's valuable comments and suggestions. We have checked our manuscript thoroughly and revised mentioned "Ptraiz-CN" into "Ptriaz-CN".
Comment 1: Despite "Conclusion" which is well-written, "Abstract" needs further rewriting to make it easier to grab the novel ideas of the research.

Reply:
We thank Reviewer 2's valuable comments and suggestions. We have revised "Abstract" in the manuscript accordingly.

Revisions made, main text (Page 2 line 23):
"N-heterocyclic Carbenes, as one traditional reactive or binding sites, have been long studied in chemical catalysis. However, their unique properties had barely been applied in functional materials. Herein, we report the design and preparation of a poly(ionic liquid)-derived porous organic polycarbene (POPcarbene) adsorbent with superior gold-capturing capability. With carbene site in the porous network as the "nanotrap", it exhibits an ultrahigh gold recovery capacity of 2.09 g/g and outstanding concentrating power for gold ion in its aqueous solution even at 100 ppb level, through a reduction-promoted adsorption process. In-depth exploration in a complex metal ion environment (Au 3+ , Pt 2+ , Cu 2+ , Ni 2+ , etc.) of an electronic waste-extraction solution proved POPcarbene adsorbent with a significant gold recovery efficiency (REE) of 99.8 %. X-ray photoelectron spectroscopy (XPS) study along with nuclear magnetic resonance (NMR) spectroscopy reveals that the high performance of the POPcarbene adsorbent results from the formation of robust metal-carbene bond in the porous polycarbene nanotraps plus the power to reduce close-by gold ions into nanoparticles. Density functional theory (DFT) calculations indicate the energetically favorable multinuclear-Au binding enhances adsorption as clusters, leading to a surprisingly high gold capacity, where the capture abilities for metals other than Au (Pt, Cu, etc.) stay predominantly at electrostatic interactions. Life cycle assessment (LCA) and cost analysis indicate that the synthesis of POPcarbene adsorbent meets the criteria for green chemistry principles and shows emerging potentials for industrial productions. These results reveal the potentials to apply "carbene chemistry" into materials science and highlight POPcarbene as rising materials for precious metal recovery, who may guide their future exploration strategies for real-life implications." Comment 2: Introduction is well-written.

Reply:
We thank Reviewer 2's encouraged comments.

Comment 3: Line 151. This method is not HADDF. This is STEM or qualitative EDS and should not be mixed up with other advanced methods.
Reply: We thank Reviewer 2 pointed out this inaccurate expression. We have changed "HADDF-STEM images" to "STEM images" in the manuscript.
Comment 4: Line 153. How did you analyze particle size of Au? From BET? when there are a porous material and particles next to each other in BET analysis, particle size and porosity size are mixed up and not reliable. Any other method was employed?
Reply: We thank Reviewer 2's valuable comments. Indeed, we used a software called ImageJ to analyze the particle size of Au particles from STEM images. Specifically, we marked more than 100 Au nanoparticles in the STEM images for Au-loaded Ptriaz-CN-A, and then used the software--ImageJ to make statistics on the particle size of Au nanoparticles (Fig. R9).

Reply:
We thank Reviewer 2's valuable comments and suggestions. In addition to Freundlich adsorption isotherm model, the Langmuir adsorption isotherm model is commonly used in adsorption experiments to fit data. The Langmuir adsorption isotherm model assumes that all adsorption sites in the adsorbent are equivalent, which forms a homogeneous surface with the same force on the surface of the adsorbent.
The Langmuir adsorption isotherm model can be described mathematically as follows: Where Ce is the equilibrium concentration (mg g -1 ), Qe is the equilibrium adsorption capacity (mg g -1 ), Qmax is the maximum adsorption amount at the time of equilibrium (mg g -1 ), kL (L mg -1 ) is the Langmuir adsorption isotherm constant. According to Reviewer 2's suggestions, we made new adsorption isotherm curves of Au 3+ and Pt 2+ adsorption by Ptriaz-CN-A, fitted by Langmuir model. They are shown in the Fig. R10 and Fig. R11. As can be seen in the figure, the isotherm fittings are largely improved for Au 3+ . Fig. 2a and Fig. S9 show the adsorption isotherms of Au 3+ and Pt 2+ on the Ptriaz-CN-A. It can be seen that the adsorption amounts increase with the increasing Au 3+ /Pt 2+ concentration, and the adsorption capacity could greatly increase and reach its maximum at high concentrations. The Ptriaz-CN-A seized significant amounts of gold ions with an uptake capacity of as high as 2.09 g/g, which outperforms most reported porous organic polymers (see Table S1). By contrast, in the same experiment, the adsorption amount of platinum in Ptriaz-CN-A is 1.62 g/g, nearly 78 % of the gold case. Moreover, the Freundlich adsorption isotherm model and Langmuir adsorption isotherm model were employed to analyze the adsorption isotherm data. The results indicated that the adsorption isotherms were well-fitted with the Langmuir model ( Fig. 2a, Fig. S10 & Fig. S11), which yielding the linear correlation coefficients as high as 0.988 for Au 3+ and 0.88 for Pt 2+ , respectively.    same force on the surface of the adsorbent. The Langmuir adsorption isotherm model can be described mathematically as follows:

Revisions made, supplementary information (Page 12 line 219): new
Where Ce is the equilibrium concentration (mg g -1 ), Qe is the equilibrium adsorption capacity (mg g -1 ), Qmax is the maximum adsorption amount at the time of equilibrium (mg g -1 ), kL (L mg -1 ) is the Langmuir adsorption isotherm constant."  (Fig. R12). Since the as-synthesized Ptriaz-CN-A is a cationic adsorbent, it exhibits strong electrostatic interactions with anionic metal ions like AuCl4and PtCl4 2-. Meanwhile, PtCl4 2is more negatively charged comparing with AuCl4and it consequently displayed stronger electrostatic interaction with the cationic triazolium rings. Besides, PtCl4 2and the C5 proton of Ptriaz-CN-A is a Lewis pair (H···Pt 2+ ) and also exhibited strong interactions.
To investigate the interaction and adsorption ability of Ptriaz-CN-A towards silver ions, the adsorption isotherm experiment was performed. Typically, 5 mg of Ptriaz-CN-A was placed in 10 mL aqueous solutions with varying Ag + concentrations (100-900 ppm). The solutions were stirred for 24 h at 600 rpm under dark conditions to achieve adsorption equilibrium. The solutions were filtered through a 0.45 μm syringe filter units and the filtrate was analyzed via ICP-OES to determine the residual Ag + concentrations. Fig. R13 showed the adsorption isotherm of Ag + on Ptriaz-CN-A, and the adsorption isotherm data was analyzed in detail by the Langmuir adsorption isotherm model. It can be seen that the adsorption amount increases with the increasing Ag + concentration, and the adsorption capacity reaches its maximum at high concentration and achieves adsorption equilibrium. The adsorption amount of silver in Ptriaz-CN-A is approximately 495 mg/g, far less than Au and Pt. The above results proved that the interaction between Ptriaz-CN-A and silver was weak, resulting in a very low adsorption capacity.
For the selective adsorption of Au 3+ / Pt 2+ / Ag + by the Ptriaz-CN-A, since the metal salts of gold and platinum we selected are HAuCl4 and K2PtCl4, and the metal salt of silver is generally AgNO3, but the mixed aqueous solution of these three metal salts will produce AgCl precipitation, so subsequent selective adsorption experiments cannot be performed to determine the selective adsorption efficiency of the Ptriaz-CN-A between Au 3+ , Pt 2+ and Ag + .   Reply: We thank the Reviewer 2's valuable comments and suggestions. Firstly, we duplicated the test to get the e-waste solution. The detailed information about concentration of metals is shown in Table. R1. The results showed that there was little difference between the concentration of metal ions obtained after the duplicated leaching test. We think this can be attributed to the leaching method we chose. As mentioned in the manuscript, we used Yang's NBS/Py method to extract gold from electronic waste (Angew. Int. Ed. 56, 9331-9335 (2017).). Compared with other traditional leaching method (e.g. using aqua regia to leach gold from ewaste, and this yields an extremely acidic gold-containing leachate), this NBS/Py method has been proved to exhibit significant Au leaching preference over other traditional cheap metals (e.g. Cu, Ni, Mg, Zn and so on). This is why the content of Cu and Ni in our electronic wastewater is not very high. And the concentrations of metal ions in the e-waste water obtained in our experiments are in consistent with the other reports (ACS Sustain. Chem. Eng. 10(30), 9719-9731 (2022)., Chem. Mater. 32, 5343-5349 (2020).), which also used this NBS/Py method to get leaching solution. Based on the above discussion, we think this state-of-the-art leaching method is the best choice, and the high leaching selectivity for Au easily afforded the straightforward adsorption for Au 3+ in e-waste solution with high efficiency.
And in Line 167 (previous manuscript), we believe there is a misunderstanding. Here we set the initial concentration of Au 3+ solution to 100 ppm not to carry out the gold extraction experiment of authentic electronic wastewater or the selective adsorption experiment of metal ions, but to carry out the adsorption kinetic experiment, and study the influence of solutions' pH on the adsorption process. The adsorption kinetics experiments of Ptriaz-CN-A were conducted by collecting samples at different time intervals. The adsorbent was then filtered by 0.45 μm syringe filter units and the remaining Au 3+ concentrations in solution were determined by ICP-OES, so as to calculate the removal rate of Au 3+ by the adsorbent. Commonly, most studies (ACS Sustain. Chem. Eng. 10(30), 9719-9731 (2022)., ACS Appl. Mater. Interfaces 12, 30474-30482 (2020)., ACS Appl. Mater. Interfaces 14, 11803-11812 (2022). Chem. Eng. J. 410, 128360 (2021).) also set the initial concentration of metal ion solution as 100 ppm. Table. S4 was added in supplementary information. Reply: We thank the Reviewer 2's valuable comments. In the cationic 1,2,4-triazolium ring, the C5 proton is highly active and undergoes easier deprotonation. In an aqueous solution, the proton exchange process between Ptriaz-CN adsorbent and H2O continues constantly. When the pH of the solution is 2, 4 and 7, the proton exchange process will not be affected, and no other negative ions in the solution can compete with AuCl4for electrostatic interactions with Ptriaz-CN adsorbent. So, when the pH is below 7, the adsorption process of Ptriaz-CN adsorbent on AuCl4is not affected by the pH of the solution. When the pH is above 7, the excessive OHis prone to compete for the adsorption sites, thus, the recovery efficiency of Ptriaz-CN-A on AuCl4is affected slightly. However, Fig. 2g was added in main text to replace previous Figure.   Table. S3 was added in supplementary information. Reply: Thanks for Reviewer 2's valuable comments, we have made a detailed analysis of the CPU pins according to your suggestion. The EDX analysis and SEM mapping (Fig. R15) showed that the CPU pins were mainly composed of inner Cu cores (> 99.0 wt%) and outer Au coatings (> 95.0 wt%). In addition, EDX analysis evidenced the presence of Ni (> 4.0 wt%) in the CPU pins, which mainly served as a middle layer between Au and Cu to promote the stable adhesion of Au to the Cu core (Appl. Surf. Sci. 185, 289 (2002).), and there are also trace amounts of Pt (0.24 wt%) in the inner cores.

Revisions made, main text (Page 13 line 264)
: "X-ray spectroscopy (EDS) mapping for one single pin in the scrap CPU ( Fig. 3a & Fig. S16)."   Compared with traditional alkali cyanide leaching method, who exhibited few obvious drawbacks (e.g., its well-known lethal toxicity, risky explosiveness, high energy consumption, etc.), this NBS/Py method shows significant Au leaching preference over other metals and achieves an approximately 90 % leaching efficiency of Au at room temperature with a nearly neutral pH. Moreover, the minimum dose of NBS/Py is as low as 10 mM, which exhibits low toxicity towards aquatic creatures. In our research, the metal leaching solution was prepared by mixing 0.966 µL of pyridine and 750 mg of NBS in 120 mL of DI water. 0.21 g of the metal scraps were put into the solution and the mixture was let stand for 4 days. Generally, this strategy shows low environmental impact with negligible cytotoxicity. This NBS/Py method is important as it provides a simple, ecofriendly, feasible option to leach gold, by reducing the total chemical waste and energy load. That's why we chose it.

Revisions made, main text (Page 13 line 267):
"Yang's leaching route employs mild and environmentally benign conditions (neutral pH, and chemicals of low toxicity) that can efficiently oxidize Au 0 in electronic waste into Au 3+ in a high yield and selectivity." Comment 14: Line 232. Please correct it to: "... was filtered and then acidified by HCl to pH = 2 ..." Reply: Thanks for Reviewer 2's valuable comments and suggestions. As you mentioned, we have corrected it to: "... was filtered and then acidified by HCl to pH = 2 ..." in our revised manuscript.

Revisions made, main text (Page 14 line 270):
"…the resulting e-waste solution was filtered and then acidified by HCl to pH = 2…"

Comment 15: Line 237. Please repeat the test with high concentration of Cu and Ni, since usually in e-waste leaching solutions, Cu and Ni are in high concentrations.
Reply: Thanks for Reviewer 2's valuable comments and suggestions. As you mentioned in the authentic e-waste solution (Comment 8), we have duplicated the test again to get the e-waste solution. The detailed information about concentration of metal ions in e-waste leaching solution is shown in the Table. R1 The results showed that there was little difference between the concentration of metals obtained after the duplicated leaching test. We think this is attributed to the leaching method we chose. As mentioned in the manuscript, we used Yang's NBS/Py method to extract gold from electronic waste (Angew. Int. Ed. 56, 9331-9335 (2017).). Compared with other traditional leaching method (e.g., using aqua regia to leach gold from e-waste, and this yields an extremely acidic gold-containing leachate), this NBS/Py method exhibited significant Au leaching preference over other traditional cheap metals (e.g. Cu, Ni and Zn). This is why the content of Cu and Ni in our e-waste solution is not very high. And the concentrations of metal ions in the e-waste water obtained in our experiments are in consistent with the other studies (ACS Sustain. Chem. Eng. 10(30), 9719-9731 (2022)., Chem. Mater. 32, 5343-5349 (2020).), which also used this NBS/Py method to get leaching solutions. Based on the above discussion, we think this state-of-the-art leaching method is the best choice, and the high leaching selectivity for Au can easily afford the straightforward adsorption for Au 3+ in e-waste solution with high efficiency.
In such leaching solution, Cu 2+ is the major metal component (357.56 ppm), and Au 3+ is the minority (2.235 ppm). Still, Ptriaz-CN-A displayed a satisfied selectivity for Au 3+ (REE ⁓ 92.75 %) and Pt 2+ (REE ⁓ 36.97 %) in the presence of abundant Cu 2+ and other metal ions. By contrast, Cu 2+ concentration in e-waste water is only reduced approximately by 8 ppm to 349.22 ppm, giving an ultralow REE of 2.33 % (Fig. R16). Table. S4 was added in supplementary information.  Element e-waste solutions by carbene chemistry (metal-carbene binding affinity, evidenced by XPS and NMR analysis.), which, to the best of our knowledge, have never been investigated. It can be foreseen that such findings can expand "carbene chemistry" further to materials science. Additionally, the uptake of our material--POPcarbene adsorbent reached 2090 mg/g for Au, largely exceeding the one in the literature (428.6 mg/g for Pd).

Comment 2: The morphology modulation of Ptriaz-CN-A is not fine enough and the resulting specific surface area is low.
Reply: Thanks for Reviewer 3's valuable comments and suggestions. We have improved the preparation process for Ptriaz-CN-A. Specifically, we extended the time of crosslinking & solvent exchange process from 2 hours to 4 hours, and in the process of supercritical CO2 drying process, the liquid CO2 cleaning time was extended to 10 h to completely remove the residual DMF in the Ptriaz-CN-A. After certain adjustment of preparation methods, the specific surface area of Ptriaz-CN-A can reach 332 m 2 /g (Fig. R3) and the morphology of modulated Ptriaz-CN-A is shown in the Fig. R18.

Revisions made, main text (Page 7 line 133):
"Hence, N2 sorption (at 77 K) measurement was performed to access the pore characteristics of the Ptriaz-CN-A, which based on Brunauer-Emmett-Teller (BET) equation. Its specific BET surface area is calculated to be 332 m 2 g -1 (Fig. 1c), which is fairly acceptable for porous materials prepared free of external templates." Fig. 1c was added in main text to replace previous figure.

Comment 3: As for the gold extraction from CPU, the mechanism of elution regeneration is needed for further investigation. Is there some variation in the microstructure of recycled materials? In addition, the number of cycling recovery is too low and it is recommended to increase to more than 10.
Reply: Thanks for Reviewer 3's valuable comments and suggestions. In this study, we used acidic thiourea solution to regenerate Au-loaded Ptriaz-CN-A in desorption process. In a typical desorption experiment, the Ptriaz  The adsorbent was then filtered and washed with DI water, and subsequently dried in convection oven at 60 ℃ for 12 h before being subjected to another adsorption process. For the mechanism of elution regeneration, thiourea ((NH2)2CS) used as a gold extracting agent has shown excellent performance. (Hydrometallurgy 115-116, 30-51 (2012).). Typically, in acidic conditions, thiourea dissolves gold, forming a cationic complex; the reaction is rapid and gold extraction efficiencies of up to 99 % can be achieved. The anodic reaction follows the equation: As for the Ptriaz-CN-A, C5-proton is highly active and a reversible proton exchange take place between H2O and Ptriaz-CN-A constantly in aqueous solutions. During the adsorption process, the N-heterocyclic carbene sites can coordinate with gold ions, followed by C-Au bond formation and reductive immobilization of Au 3+ into Au 0 . When thiourea stripped gold from N-heterocyclic moiety, the N-heterocyclic carbene sites will catch protons in the aqueous solution and recharged, converting back to the original state (Fig. R19). Moreover, the variations in the microstructure of recycled Ptriaz-CN-A after 11 cycles were investigated by SEM characterization and the BET surface area was tested by N2 sorption (at 77 K) measurement. As shown in Fig. R20, before adsorption, a great number of particles can be clearly observed as a secondary structure motif, which stems from NH3-triggered, intramolecularly crosslinked Ptriaz chains. After the 11th adsorption-desorption cycles, there existed some variations in the microstructure of recycled Ptriaz-CN-A, whose particles appear to aggregate and the boundaries between particles also get vague. Furthermore, the BET surface area of Ptriaz-CN-A was reduced to 53 m 2 g -1 . (Fig. R21  Furthermore, the reusability of the material is of vitality for practical applications. Multiple adsorption-desorption experiments were performed to evaluate Ptriaz-CN-A's performance according to the Reviewer 3's valuable suggestions. Fig. R17 shows the results of reusability of Ptriaz-CN-A for Au 3+ recovery (11 cycles). The results demonstrate that the recovery efficiencies for the Ptriaz-CN-A were all above 95 % after six cycles, indicating its excellent reusability. The recovery efficiency decreased continuously in the subsequent cycles owing to the loss of the free spaces of adsorbents due to the continuous enrichment of the reduced Au nanoparticles in the networks of Ptriaz-CN-A. Fig. 3d was added in main text to replace previous Figure.  Fig. 3d shows the results of 11 cycles of the reusability of Ptriaz-CN-A for Au 3+ recovery. The results demonstrate that the recovery efficiencies for the Ptriaz-CN-A were all above 95 % after six cycles, indicating its excellent reusability. The REE decreased continuously in the subsequent cycles owing to the loss of the free spaces of adsorbents due to the continuous enrichment of the reduced Au nanoparticles in the networks of Ptriaz-CN-A. Furthermore, the regenerated Ptriaz-CN-A still exhibits more than 60 % elution efficiency (EEE) for Au 3+ after eleven adsorption-desorption cycles (Fig. S21). suggested by Reviewer 3, since most of the recovered gold particles are stacked layer by layer and aggregated into large nanoparticles, it is not possible to analyze its lattice and diffraction by HRTEM characterization. Based on above analysis, we supplemented the STEM and elemental mapping characterization (Fig. R23) to verify the purity of recovered gold particles. Additionally, more characterization about SEM and TEM of the recovered gold particles are shown in Fig. R24 and Fig. R25.

Revisions made, main text (Page 14 line 276):
"In order to study the property of recovered gold nanoparticles after the extraction from e-waste solution, the Ptriaz-CN-A (Au) was calcined in air at 900 ℃, and the obtained powder was subsequently immersed in concentrated hydrochloric acid (aq.). Despite high concentrations of competing ions in e-waste solution (e.g. Cu 2+ , Ni 2+ ), these competitive ions are easily dissolved in the concentrated hydrochloric acid after soaking. After that, the treated powders were subsequently washed by deionized water and dried in the oven. Obviously, the resulting material reveals pure solid gold particle signals in the PXRD spectrum (Fig. S17), the XRD patterns of resulting material displayed distinct peaks that fit well with the metallic gold. Furthermore, more characterizations about SEM, TEM, STEM and elemental mapping of the recovered gold particles were also performed ( Fig. S18 to Fig. S20).  Fig.  S18 was added in supplementary information.

Comment 6: Considering extremely high amount of Cu over Au, the energy changes of Cu need to be added in DFT calculations.
Reply: We thank Reviewer 3's valuable comments and suggestions. We supplemented DFT calculations to simulate adsorption and interactions between Ptriaz-CN-A model unit and CuCl2, and the corresponding energy difference (ΔE) in the reaction process is shown in Fig. R26. In detail, the 1,2,4-triazolium units firstly adsorb neighboring CuCl2 from the aqueous solution, and the DFT calculated energy difference (ΔE1) is -0.54 eV (step 1). Then two HCl molecules are removed from acid-base neutralization between the protons in C5 position of 1,2,4-triazolium units with Clin CuCl2, yet the energy difference (ΔE) in the reaction process is positive (2.45 eV) during the removal of two Cland the formation of metal-carbene bond. This result indicates the thermodynamical infeasibility of the Cucarbene bond formation between 1,2,4-triazolium and copper ion at room temperature in this specific case.

Revisions made, main text (Page 19 line 381):
"And the positive energy difference was also observed for CuCl2 adsorption in the reaction process of Cu-carbene bond formation (Fig. S35)." Fig.  S35 was added in supplementary information.

Comment 7:
The saturated adsorption capacity of this manuscript is 2.09 g/g. If the author carefully read more recent literatures, it can be found that adsorption performance of the proposed POPcarbene adsorbent is not superior in the field of gold recovery.
Reply: Thanks for Reviewer 3's valuable comments and suggestions. We have summarized high-quality literatures of powder adsorbents for gold recovery from electronic wastewater in the past three years. Indeed, there are few porous polymer materials with excellent adsorption performance in the field of gold recovery. Although our Ptriaz-CN-A is not superior in this field, the Ptriaz-CN-A has excellent adsorption performance among many powder adsorbents (Table. R3).

Comment 8: The preparation of Ptriaz-CN-A powder material is rather complicated and expensive, lack of superiority in the engineering application.
Reply: Thanks for Reviewer 3's valuable comments. As we mentioned in our manuscript, the preparation of Ptriaz-CN-A include three steps in polymer synthesis (monomer synthesis, polymerization and anion exchange) and two steps in the posttreatment (TIP process and ammonia treatment/solvent exchange), followed by freeze drying (Fig. R27). All procedures are conducted under mild conditions (temperature≦70 °C, under atmospheric pressure). In comparison, porous polymer materials with high gold uptakes mostly require harsh conditions during synthesis (solvothermal methods, ≥100 °C for several days, e.g. certain COFs), or complicated synthesis and purification steps (monomer modifications, column chromatography or re-crystallization, e.g. certain COFs and PAFs). Nevertheless, the preparation of Ptriaz-CN-A is easy-to-handle and do not need sophisticated synthesis skills. Also, these adsorbents are highly repeatable in fabrication. In general, we believe the preparation of Ptriaz-CN-A is a decent method for adsorbent fabrication and can be used for high-capacity gold uptake.
We also did a thorough life cycle assessment and cost analysis for the production of Ptriaz-CN-A. And please see the Additional

Supplementary Information (Life Cycle Assessment and Cost Analysis)
for detailed data and analysis. Furthermore, our data collection and analysis are considered on a laboratory scale. As shown in Fig. R7, the results indicate that there is much room to reduce the environmental impact and cost of Ptriaz-CN-A production when they are produced at scale, and show emerging potentials for industrial productions.  Fig. 6a and Fig.6b was added in main text. Production cost distribution of raw materials, electricity in each step for the synthesis of 1 g Ptriaz-CN-A at different preparation scales. (Scale-1: small dose feeding, data based on our current research; Scale-2: big dose feeding, data based on the maximum production scale of the laboratory.) (Fig. 6 in revised Fig. R28 Proposed mechanism of Au 3+ adsorption-reduction on the Ptriaz-CN-A (bigger geometries). (Fig. S28 in revised supplementary information).  Reply: Thanks for the Reviewer 4's valuable comments and suggestions. We apologize for the misunderstanding raised here, the mentioned "negligible energy barrier" refers specifically to the energy difference (ΔE) in reaction process. And we have corrected it into "energy difference" in the revised manuscript. But according to your constructive suggestions, we have searched transition state (TS) to the best of our ability in a relatively simple way. Considering the limited time, we used ORCA software to search TS structures from the geometry of reactants and products by Nudged Elastic Band with TS optimization (NEB-TS) analysis rather than Step-2

Ptriaz-CN-A (Pt) Relative Energy (eV)
conventional searching methods in Gaussian 09. Specifically, the basis sets and functionals we used were in consistent with them used in Gaussian 09 (TPSSh/def2-svp), and D3 keywords were added to consider the dispersion effect and perform dispersion correction. Then we optimized the structure of the speculated "reactants" and "products", and performed NEB calculation tasks to obtain the minimum energy path (MEP) of the reaction. Next, we extracted the structure ST1 with the highest energy point in the MEP to be the TS structure. After that, the obtained TS geometry was optimized by Gaussian and calculated to get the single point energy. Finally, we used the difference between the electron energy of the TS structure and the electron energy of the reactant structure as the energy barrier during the reaction process. The relative MEP figures and energy profiles of reaction process for both PtCl4 2and AuCl4are shown in Fig. R31 to Fig. R34.
Typically, for the reaction process of AuCl4 -, the energy of the TS is a little lower than that of reactant, and this may be a negative TS. This means that the reaction process of AuCl4from Step-1 to    Reply: Thanks for Reviewer 4's valuable suggestions. Since AuCl4and H3O + have closed shell structures, the spin multiplicity of the overall structure is equal to that of the Ptriaz-CN-A model unit. In the current work, we used Gaussian 09 software to perform DFT calculation for Ptriaz-CN-A model unit using functional and basis set of TPSSH/def2-SVP. The detailed spin multiplicity for the Ptriaz-CN-A unit and the corresponding total energy are shown in the Table R4. It is clearly that when the spin multiplicity is 1, Ptriaz-CN-A model unit has the lowest energy, indicating a closed shell structure for all electrons in the unit model. For the adsorption of AuCl4 -, in Step-1 and Step-2, the number of electrons in the whole system is even and it is a closed shell system, so the spin multiplicity is 1; in Step-3 and Step-4, the whole system has an odd number of electrons and is an open-shell system, so the spin multiplicity is 2. For the adsorption of PtCl4 2-, the system has an even number of electrons throughout the adsorption process, so it is a closed shell structure with a spin multiplicity of 1. (1) When spin multiplicity = 1, there is no spin contamination, so no S**2 can be found.

Comment 8:
Spin Density values might be given for the atoms (especially gold atoms) on the structure. These values tell us where unpaired electrons are located in the system.

Reply:
We thank Reviewer 4's valuable comments. When the spin multiplicity is 1 (Step-1 and Step-2), the numbers of α and β electrons are the same, and therefore the total spin density is 0. For Step-3, the spin multiplicity for system is 2, there will be an unpaired electron in the system. As shown in the Fig. R35, the unpaired electron is mainly concentrated on the triazine ring, but not around the Au atom. This may be ascribed to the adsorption-reduction process and the formation of metal-carbene bond for capturing Au ions. In Step-1, the Ptriaz-CN-A model unit interacts with free AuCl4 -, and the valance of Au is +3 at this time. From Step-1 to Step-2, corresponding to the process of deprotonation and coordination with metal ions to form C-Au bond, the reductive elimination between 1,2,4triazolium and AuCl4takes place and stepwise reduces AuCl4to AuCl2by releasing HCl molecules, and the valence of Au is +1 at this time. Furthermore, the AuCl2continues to be reduced by releasing HCl molecules, and finally becomes stable Au atom state (the valance of Au is 0 at this time), which is coordinated with carbene site. The unpaired electron is mainly concentrating on the triazine ring due to the coupling of σ-π orbitals between Au and the triazine ring, and the electrons are more stable on the π orbital above and below the triazine ring. (2) When the spin multiplicity is 2, conceptual density functional theory (CDFT), we can directly calculate the wave function and energy of N, N+1, N-1 electronic states, and then obtain the parameters.

Revisions made, main text (Page 18 line 371):
"Furthermore, some critical values (chemical hardness, chemical potential…), molecular electrostatic potential diagrams and HOMO/LUMO representations in adsorption process were also calculated by Multiwfn software (Table S5, Table S6, Fig. S31 to Fig. S34)." Table S5 was added in supplementary information.  Table S6 was added in supplementary information. Reply: Thanks for Reviewer 4's valuable comments and suggestions. As for the geometry of Ptriaz-CN-A model unit, we firstly constructed the original model according to the chemical structure of Ptriaz-CN-A, which obtained from the results of experiment (Fig. R36). The model was then geometrically optimized by TPSSH/def2-SVP (opt+freq) using Gaussian 09, and the obtained structure showed successful convergence to the right energy and force threshold, and no imaginary frequency for the obtained structure in frequency calculation. Subsequently, vibrational infrared frequency was calculated, and it was compared with the experimental values characterize the geometries obtained by DFT calculations. As shown in Fig. R37, it is clearly that the distinct stretching bands of Ptriaz-CN-A and simulated Ptriaz-CN-A model unit are basically consistent. Based on the above discussion, we can verify that the geometry of Ptriaz-CN-A model unit obtained from DFT calculation is optimized. In our research, the adsorption-reduction process and the formation of metal-carbene bond for capturing gold was reckoned. As for the coordination numbers of carbene sites between Au, we established a coordination model between Ptriaz-CN-A and Au based on NMR results (Fig. R38), in which two carbene sites can stabilize one Au atom. The model was then geometrically optimized by TPSSH/def2-SVP (opt+freq) in Gaussian 09, and output results showed successful convergence of simulation results and no imaginary frequency in frequency calculation (Fig. R39).

Comment 12:
What are the convergence criteria in calculations? Gradients of root-mean-square (rms) displacement, max displacement, rms force, max force and the self-consistent field (SCF) convergence.
Reply: Thanks for Reviewer 4's valuable comments. In DFT calculations, it has four parameters to judge the convergence of geometric optimization. Using default convergence settings (convergence on RMS density matrix=1.00D-08 within 128 cycles, convergence on MAX density matrix=1.00D-06, and convergence on energy=1.00D-06), the four parameters are: (1) gradients of root-mean-square (rms) displacement < 0.00120; (2) max displacement < 0.00180; (3) rms force < 0.00030; (4) max force < 0.00045.  Consequently, as shown in the Table R7, we found that the charge-density becomes smaller and closer to zero for the Au atoms when linked with Nheterocyclic moiety, revealing that charge transfer from the carbene site to Au atom, leading to the formation of strong Au-carbene chemical bond during structural accommodation. These results verify that the Nheterocyclic moiety acts as an electron-donating group that increases the electron density of Au atoms by forming C-Au bond and can further stabilize gold nanoparticles generated thereafter, which is in consistent with our experimental findings. Step-1 to Step-4 during adsorption. Step-1 Step-2 Step-3 Step-4

Reply:
We thank Reviewer 4's valuable suggestions. We have applied solvent effect (water) to perform DFT calculations and compare energy difference throughout the whole adsorption process for Au/Pt with present values. The detailed energy values of each model unit and the energy difference of adsorption process before/after adding solvent effect are shown in the Table R8 to Table R11. It is clear that there is marginal change for the total energy of model unit when we take solvent effect in consideration, but the trend of energy difference is the same as before. This result indicates that the thermodynamical infeasibility of the formation of Pt-carbene bond between 1,2,4-triazolium and Pt ion at room temperature, no matter whether or not solvent effect is applied. In comparison, a stepwise exothermic process is observed from the calculated AuCl4adsorption process. So, the results after adding solvent effects are in consistent with our previous conclusions. 19 line 396): "Moreover, no matter whether or not solvent effect is applied in DFT calculations, the trend of the reaction process is the same ( Table S6 to Table S10)." Table S7 was added in supplementary information.  Table S8 was added in supplementary information.  Table S9 was added in supplementary information.  Table S10 was added in supplementary information.

Comment 16: Energy values should be included thermal energy corrections.
Reply: Thanks for the Reviewer 4's comments and suggestions. We have extracted all energy-relevant results (including the thermal energy corrections) and these results are listed below.